Electromagnets are commonly used in industry to lift and move heavy metal objects, such as scrap metal. In general, an electromagnet consists of a wire coil (the "magnet winding") wound around an iron core. A magnetic field is generated in the electromagnet when electric current flows through the magnet winding. Typically, electrical power is supplied to the electromagnet from a DC generator.
Prior control systems for electromagnets have primarily used two different approaches to controlling the electromagnet. According to a first approach, switches having high current contactors are connected between the DC generator and the electromagnet to allow an operator to selectively open and close the circuit between the DC generator and electromagnet to power the electromagnet on or off. This approach has a number of problems, including that the arrangement is prone to voltage surges and current arcing which cause damage to the magnet winding, switch contactors and insulation.
A second approach to controlling the electromagnet is to controllably vary the excitation voltage applied to the field winding of the DC generator. With this second approach, the DC generator and electromagnet can remain connected at all times. Power to the electromagnet is instead interrupted by simply removing the excitation voltage from the generator field winding. Also, the direction of the electromagnet's magnetic field can be reversed by reversing the polarity of the voltage applied to the generator field winding. This approach has the advantage that a much lower current is applied to the generator field winding as compared to the electromagnet, so that switching power on and off to the generator field winding is less likely to cause surges and arcing that damages switches, insulation and wiring and also less costly control circuit components can be used. Further, the electromagnet's power can be discharged back to the DC generator's armature winding when switched off, completely eliminating high voltage surges otherwise experienced when the magnet generator circuit is opened and the magnetic field collapses.
One example of a prior art control system using the second approach is described in Yorkey, U.S. Pat. No. 2,257,361.
Another example of a prior art control system using the second approach is the SofTouch.RTM. Lifting Magnet Controller marketed by International Electrical Systems, Inc. of Lumberton, Tex.
Although providing the advantages from use of the second approach, the SofTouch.RTM. Lifting Magnet Controller and like prior art control systems still have several disadvantages. These disadvantages include the use of variable resistors to set the voltages applied to the generator field winding during the lift and drop cycles of the electromagnet operation cycle. Suitable variable resistors for producing the required voltages for the generator field winding are heavy and bulky. Further, the variable resistors used in the control system often have to be swapped according to the rating of the particular DC generator and electromagnet for use in a particular installation. Also, an additional AC generator must be ordered according to the size of the system and physically installed in the crane in order to provide the AC power required to excite the primary DC generator.
Additionally, prior art magnet control systems have been rigidly mounted in order to adequately support heavy, high current electrical components used in the control systems, such as variable resistors and high current switch contactors. However, the control systems typically are installed in environments subject to large amount of vibration (e.g., from a diesel engine or other motor that drives the DC generator). This can cause relay switches used in the control systems to unexpectedly trip.
Further, prior art magnet control systems historically have operated at very high voltages and currents requiring suitable insulation of their wiring paths, and could not therefore be manufactured on a printed circuit board.
The present invention overcomes these and other disadvantages of prior art magnet generator controllers. A magnet generator control system in an embodiment of the invention illustrated herein (the "illustrated magnet controller") overcomes the disadvantages of variable resistors used in the SofTouch.RTM. Lifting Magnet Controller and like control systems by providing a plug-in connector-selectable variable voltage array for selection of the voltages applied to the generator field winding. This variable voltage array includes an array of connector receptacles. Each of the connector receptacles have conductor terminals that are connected to different sets of transformer taps, such that successive connector receptacles provide discrete voltages in increasing order along the array. A plug-in connector on a cable can be placed in a desired one of the connector receptacles to select the connector receptacle's respective discrete voltage. The illustrated magnet controller provides separate variable voltage arrays for selecting each of an extra lift voltage, a lift voltage and a drop voltage. The variable voltage arrays can be readily implemented on a printed circuit board, which in the illustrated magnet controller is supported on rubber mounts for vibration isolation.
The illustrated magnet controller also provides a plug-in connector-selectable master function switch that allows selection of normal run cycle operation, or exclusive-selection of one of the modes of the normal run cycle. In the normal run cycle operation, the illustrated magnet controller sequences through the modes in set time intervals. Alternatively, exclusive operation in a single mode can be selected to more easily and safely set the voltage applied to the generator field winding in that mode.
The illustrated magnet controller further uses an AC power inverter powered by a DC battery to supply the AC excitation voltage for the generator field winding suitable for various size electromagnet systems. This eliminates the need for an additional AC generator sized specifically to the electromagnet system in which the controller is installed.
Additional features and advantages of the invention will be made apparent from the following detailed description of an illustrated embodiment which proceeds with reference to the accompanying drawings.